1. Field of the Invention
The present invention relates generally to coring tools and, more particularly, to a coring tool which provides a more accurate determination of the gas and liquids in the core even when the core is taken at significant depths and high pressure formations.
2. Description of the Background
The goal behind a pressure coring tool is to bring an in situ sample of the core to surface. Ideally, the core would still contain all of the gas and various fluids that the core originally contained when captured at reservoir pressure. If the core samples are the same as they were when captured, subsequent measurements can be used to estimate the reserve gas in the formation.
However, problems arise with conventional coring tools because many presently produced formations are at 10,000 feet and greater where pressures range from 7500-12000 psi.
One approach in the past to this problem has been to attempt to create a chamber and a valve that are capable of holding the high bottom hole pressures, as well as the fluids and gases, as the core is retrieved to the surface from 10000 feet or more in a well bore. However, such attempts have been unsuccessful due to numerous problems.
For example, after retrieval, the coring tool contained the very high and dangerous pressure on the surface. This makes the coring tool difficult to handle and potentially quite dangerous to the drilling crew as the core is removed from the well.
In some cases, the valve may malfunction and is kept open or partially open by the core itself, thereby losing a significant amount of fluids and gases. A malfunctioning valve might also suddenly release pressure at the surface, which could produce a dangerous high pressure spray.
This prior art design requires a high pressure chamber and high pressure valve, which results in greatly limiting the volume of the retrieved core so that only a one to two inch diameter core might be obtained from an eight and one half inch borehole. As well, the cores from such tools were very short. Smaller cores are inherently less desirable and/or reliable for calculations.
Another approach has been to use estimation calculations, which were successful at shallower depths, e.g., for relatively shallow coal bed methane (CBM) formations. However, over the past years, new sources of natural gas formations have been developed at considerably greater depths and pressures for which the estimation calculations are no longer accurate. For example, a present trend involves producing shale gas from the deeper formations. To determine the amount of natural gas contained in the CBM formations, the core was put into canisters after the core was brought to surface. The canisters were sealed but left at atmospheric pressure to allow all of the gas to “bleed” out. The gas was then measured. Through specifically derived calculations, the amount of gas the reservoir contained could be determined.
Wire line coring was a integral part of this equation because after cutting the core, the core could be retrieved to the surface within minutes, therefore minimizing the gas that was lost during the trip out of the hole. The amount of gas lost from the time the core was subjected to a lesser pressure than reservoir pressure (once tripping out of the well bore had begun) could only be estimated from calculations. As well, in coal cores, the gas “bleeds” out the core slowly. So when combined with the fast tripping of wire line coring, the back calculations were very accurate.
When the exploration of shale gas began, the gas community thought it would be possible to apply the same calculations to shale and the problem would be solved. There were two major issues: (1) The new shale gas formations were at much greater depths than the shallow coal seams of CBM. This meant that the differential pressure from reservoir pressure to atmospheric was much greater, which forced more gas out before the core was at surface, and (2) Most of the new shale gas formations contained as much as 95% “free gas”. This term means just what it suggests, 95% of the gas is lost due to the pressure decrease while tripping out of the hole, so it only leaves 5% to be analyzed. Back calculating with any degree of accuracy from the 5% content remaining in the core is virtually impossible.
General background prior art patents include the following:
United States Patent Application 2012/0037427 to Douglas Kinsella, filed Aug. 10, 2010, discloses a drill string assembly that has the capability of operating in well bores that range in hole size from seven to eight inches in diameter and is incorporated herein by reference. The assembly is used to obtain a large core sample size that is equal to three and one-half inches in diameter and up to ninety feet in length in a single core run. This assembly will be operated with a drill string (i.e. drill pipe) that is capable of being used on standard drilling rigs, which may be used to handle API style drill pipe to conduct coring/drilling operations. The coring tool is comprised of an inner barrel for receiving the core sample.
U.S. Pat. No. 6,736,224 to Douglas Kinsella, issued May 18, 2004, discloses a wellbore assembly that is operable in wellbores in the range of six to six and one-half inches for obtaining large diameter cores, e.g., cores greater than or equal to two and seven-eighths inches in diameter and is incorporated herein by reference. The wellbore assembly may preferably be utilized with drill pipe so that standard drilling rigs may be utilized in drilling and coring operations therewith. The drill pipe in accord with the present invention may be formed by modifying standard API drill pipe such as API four and one-half inch IF (Internal Flush) drill pipe in a special manner that renders the drill pipe still suitable for the type of drilling operations of interest and also suitable for handling by any drilling rig capable of using standard API drill pipe. Alternatively, the drill piper may be initially manufactured in accord with the specifications of the present invention. The coring tool preferably comprises an inner core barrel for receiving the core and, in a presently preferred embodiment, may be sized to obtain a core having an outer diameter from about three to three and one-half inches.
Accordingly, it would be desirable to provide a pressure coring tool that provides improved capture of gas and fluids present when the core is initially taken at down hole depth and pressure. Consequently, there remains a long felt need for an improved coring tool. Those skilled in the art have long sought and will appreciate the present invention which addresses these and other problems.